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Copy number variation is variation in the number of copies of a particular DNA segment among genomes. A deletion reduces the count; a duplication increases it. For an segment on a non-sex chromosome normally present once on each of two homologous chromosomes, a diploid cell usually has two copies, whereas one additional tandem copy produces a total of three.

The number must be interpreted against a specified baseline. In an ideal comparison with equal sample normalization, three copies versus two give a DNA abundance ratio of $3/2$, or ${\log }_2(3/2)\approx 0.585$ on a logarithmic readout. This arithmetic does not imply that gene expression must rise by the same proportion.

Genome-wide DNA hybridization experiments in 2004 found substantial large-scale copy number variation among otherwise healthy individuals. Such variation is part of ordinary human genetic diversity as well as a possible contributor to disease.^[ref]

Jonathan Sebat led a 2004 study demonstrating widespread large-scale copy number variation in the human genome. Comparing genomic DNA by hybridization detected regions whose copy numbers differed among individuals; many differences were inherited.^[ref]

A person can therefore differ from another not only at individual nucleotides, but also in how many copies of a substantial DNA segment are present. Comparing signal intensities supplied evidence for dosage differences that sequence substitutions alone would miss. The result helped make structural variation a routine part of questions about human genetic diversity.